You are correct, Natchez Trace has some gentle grades, most don't warrant a gear shift. Now, the foothills and mountains in NC are pretty nice climbs. Lance Armstrong used most of the roads and climbs around Boone, NC for his rehab from cancer. The area I ride weekly has very few hills over 3 or 4 kilometers long. Most are short, sometimes steep "bumps" on the way to longer flat stretches. Some of the more well known climbs for "roadies" in NC are in the 900 to 1800 meter range. Many of the climbs on the Blue Ridge Parkway would be in the 700 to 1300 meter range, with an occasional peak higher.